The present invention relates to a power transfer device for use in a four-wheel drive vehicle of part-time four-wheel drive type being changed over between the state of two-wheel drive and the state of four-wheel drives, and having a center differential gear unit which distributes the drive force of an engine to the front and rear road wheels.
In conventional power transfer devices for four-wheel drive, there has been provided a part-time four-wheel drive type other than a full-time four wheel drive type. Specifically, in the vehicle of part-time four-wheel drive type, it is possible to change over between the two-wheel drive state and the four-wheel drive state. Since the vehicle of part-time four wheel-drive type is advantageous that the drive force can be positively transmitted to road wheels and the driver can enjoy driving operation, there has been a great demand for the vehicle of part-time four wheel drive type, so that the maneuverability of the vehicle has been improved.
In the vehicle of part-time four-wheel drive type, a power transfer is provided which can change over between the two-wheel drive state and the four-wheel drive state. The power transfer called a two-speed power transfer has been developed in which the low speed four-wheel drive for running on a road of a terrible condition and the high speed four-wheel drive for running on a road of normal condition, can be selected.
For example, FIG. 17 is a schematic illustration of the two-speed power transfer. The two-speed power transfer 1 comprises: the H/L changeover mechanism 3 that can change the speed of the input shaft 2 to which the drive force of an engine not illustrated is transmitted, wherein the rotation speed of the input shaft 2 can be changed over between a high speed and a low speed; the rotary shaft 4 which is provided on the production of the input shaft 2 in order to transmit the drive force to the rear wheel side not illustrated through the H/L changeover mechanism 3; and the 2/4 changeover mechanism 5 which can change over between the two-wheel drive state and the four-wheel drive state by transmitting the drive force of the rotary shaft 4 to the front wheel side not illustrated or intercepting the drive force so that the changeover can be made between the two-wheel drive state and the four wheel drive state.
The H/L changeover mechanism 3 comprises: the input gear 21 which is integrally provided to the edge portion of the input shaft 2; the input hub 22 which is coaxially mounted on the input gear 21 the output hub 23 integrally mounted on the edge portion of the rotary shaft 4 which drives the rear wheels; the tubular shaft 24 which is coaxially and rotatably provided around the rotary shaft 4; the H/L changeover hub 25 which is integrally provided to the tubular shaft 24; the H/L changeover gear 26 which is coaxially mounted on the H/L changeover hub 25; the first low speed gear 27 which is supported by the countershaft 6 provided in parallel with the input shaft 2 and the rotary shaft 4, and which engages with the input gear 21; the second low speed gear 28 which is supported by the countershaft 6 and engaged with the H/L changeover gear 26; the H/L changeover sleeve 8 which is provided around the circumference of the input hub 22, the output hub 23 and the H/L changeover hub 25, and which can engage with either of the hub 22, 23 or 25; and the H/L changeover shift fork 9 which moves the H/L changeover sleeve 8.
When the H/L sleeve 8 is set to the H position in which the H/L sleeve 8 engages astride with both the input hub 22 and the output hub 23 as illustrated in FIG. 17, the rotary shaft 4 is rotated at the same speed as the input shaft 2. On the other hand, when the H/L changeover sleeve 8 is set to the L position in which the H/L changeover sleeve 8 engages astride with both the output hub 23 and the H/L changeover hub 25, the rotary shaft 4 is rotated at a lower speed than the input shaft 2 through the gears 21, 27, 28 and 26. On the other hand, when the H/L sleeve is set to the N position in which the H/L sleeve engages only with the output hub 23, the drive force of the input shaft 2 is not transmitted to the rotary shaft 4.
The 2/4 changeover mechanism 5 comprises: the hub 31 which is mounted on the rotary shaft 4 so that it can be rotated integrally with the rotary shaft 4; the tubular shaft 32 which is coaxially and rotatably provided around the rotary shaft 4; the 2/4 changeover hub 33 which is integrally mounted on the tubular shaft 32; the 2/4 changeover sleeve 10 which is provided around the hub 31 and the 1/4 changeover hub 33 so that it can be appropriately engaged with either the hub 31 or the hub 33; and the 2/4 shift fork 11 which moves the 2/4 changeover sleeve 10. The said tubular shaft 32 is provided with the front-wheel drive force transmitting sprocket 12 which is rotated integrally with the 2/4 changeover hub 33, and the transfer chain 15 is wound around the front-wheel drive force transmitting sprocket 12 and the front-wheel drive sprocket 14 mounted on the front-wheel drive shaft 13.
When the 2/4 changeover sleeve 10 is set to the two-wheel drive position in which the 2/4 changeover sleeve 10 engages only with the hub 31 as illustrate drive force of the rotary shaft 4 is not transmitted to the front wheel drive shaft 13. On the other hand, when the 2/4 changeover sleeve 10 is set to the four-wheel drive position in which the 2/4 changeover sleeve 10 engages astride with both the hub 31 and the 2/4 changeover hub 33, the drive force of the shaft 4 is transmitted to the front wheel drive shaft 13 through the hub 31, the 2/4 changeover sleeve 10, the 2/4 changeover hub 33, the tubular shaft 32, the front wheel drive force transmitting sprocket 12, the transfer chain 15, and the front wheel drive sprocket 14. In the way described above, the four-wheel drive state can be attained.
However, it should be noted that the said conventional transfer is disadvantageous in that: a tight corner braking phenomenon peculiar to the four-wheel drive may occur when the vehicle passes through a curved path at a low speed; and further while the vehicle is running at a high speed in the state of four-wheel drive, internal circulating torque may be generated according to the difference of the diameter between the front and rear road wheels, which is caused by the difference of air pressure of tires, so that there is a fear that the parts of the drive system are damaged.
In order to solve the problems described above, the method to provide a center differential gear unit to the part-time four-wheel drive vehicle can be proposed, which center differential gear unit can distribute the drive force transmitted from the engine to the front and rear road wheels.
However, when the center differential gear unit is always activated, there will be caused such a problem that an excessive slip occurs during running of the vehicle on a low frictional road.
Therefore, in this case the center differential gear unit must be provided with a locking device so that a selection can be made between the locked state and unlocked state when necessary. Specifically, it is necessary that a selection can be made between: the two-wheel drive in the condition that the center differential gear unit is locked; the four-wheel drive in the condition that the center differential gear unit is free; and the four-wheel drive in the condition that the center differential gear unit is locked.